You don't need the API for this.
Go to Help center, open the collection, and open the article.
Click where the image should go, type / and choose Image.
Click Upload an image and pick the file, or drag it onto the box. JPG, PNG, WebP and GIF work.
The image appears in the draft. On a published article, click Publish changes to make it live.
Developers who generate articles automatically can upload through the API instead (POST /v3/uploads).
